1. Field of the Invention
The present invention relates to a liquid crystal display element of optical writing type which includes a light absorbing layer formed of an organic material having pigment dispersed therein for the purpose of making the manufacturing process easier.
2. Description of the Related Arts
The present inventors know a liquid crystal display element of optical writing type which has the following arrangement. A first transparent electrode is formed on a first glass substrate. On this first transparent electrode, a photoconductive layer is formed of a hydrogenated amorphous silicon (a-Si:H). On this photoconductive layer, a light absorbing layer is formed of a patterned thin film of carbon or metal such as silver. On this light absorbing layer, a dielectric mirror is formed of multiple layered films of titanium dioxide/silicon dioxide, zinc sulfide/magnesium fluoride, for example. On this dielectric mirror, a first orientation film is formed of a polyimide film.
On the other hand, a second transparent electrode is formed on a second glass substrate. On this second transparent electrode, a second orientation film is formed. The first and second glass substrates, on which these films are thus formed, are attached to each other through a spacer, and a liquid crystal is disposed between the first and second orientation films, so as to seal the liquid crystal layer therebetween.
Then, a projection light, that is, a light for reading out an image written in the liquid crystal layer, is applied to the liquid crystal display element from the side of the second glass substrate. The dielectric mirror is provided for the purpose of reflecting the projection light toward the outside and, at the same time, preventing the projection light from entering the photoconductive layer by reflecting the projecting light with a high reflection coefficient. The light absorbing layers are provided for preventing the projecting light from being reflected toward the photoconductive layers again and, at the same time, screen the projection light transmitted through the dielectric mirror.
That is, the projection light passes through the liquid crystal and is reflected from the dielectric mirror to the outside. Since the dielectric mirror has a reflectivity of about 95%, however, about 5% of the projection light is transmitted through the dielectric mirror. When the light is incident to the overall surface of the photoconductive layer, the image data being written by a writing light is erased. To prevent such a disadvantage, the light absorbing layers are provided between the dielectric mirror and the photoconductive layer. The light incident on the liquid crystal display element normally has an intensity of about 1 W. On the other hand, if the photoconductive layer is formed of a hydrogenated amorphous silicon (a-Si:H), light having an intensity of several tens .mu. W or more may vary the conductivity of the photoconductive layer. To avoid the variation of the conductance, it is necessary to lower the intensity of the light incident on the photoconductive layer by four to five orders of magnitude through the effect of the dielectric mirror and the light absorbing layer.
One method for lowering the intensity is necessary to enhance the reflectivity of the dielectric mirror. For this purpose, however, the dielectric mirror needs to have more layers. This results in making the dielectric mirror thicker, thereby making it difficult to apply a sufficient voltage to the liquid crystal.
The known liquid crystal display element, therefore, includes (a) light absorbing layer formed of a thin film such as carbon or metal (e.g. silver) for the purpose of lowering the intensity of the incident light by four or more digits.
The light absorbing layer formed of a film such as carbon or metal (e.g. silver) has so high conductivity that the layer needs to be patterned. Patterning of the layer makes the manufacturing process more difficult for obtaining a high resolution. Moreover, the adhesion properties between the metal film such as carbon or silver and the hydrogenated amorphous silicon (a-Si:H) are not so good that the light absorbing layer may be more easily stripped off the photoconductive layer.